1. Field of the Invention
This invention relates to microwave-absorptive composite materials for use in, for example, ships, aircrafts and other mobile objects.
2. Prior Art
Microwave-absorptive materials are broadly classified into an attenuation type in which the energy of incident wave attenuates as it passes through the absorber, and a matching type in which the amount of reflection of incident wave on the front surface of the absorber and that of reflection of wave from a reflector on the rear surface of the absorber are controlled so as to in effect eliminate the generation of reflected wave.
A typical matching-type wave absorber well known in the art comprises an absorbing layer made up of a synthetic resin or rubber sheet carrying magnetic ferrite (Fe.sub.2 O.sub.3). This type of material excels of course in wave absorption, but is rather heavy, hence unsuitable for light-weight applications and furthermore structurally weak as it is a resinous or rubber sheet simply admixed with ferrite. Attempts have been made to produce a light-weight microwave absorber from a rubber composition having therein carbon black particles dispersed therein in place of ferrite. Such an absorber, however, has been effective only in handling a narrow band of microwave frequencies. It has been known that wave absorption over wide frequency bands can be achieved by the use of multi-layered absorbers. This typically involves considerable difficulty in fabrication, however, chiefly because of the different characteristic properties of the different layers. The outermost layer usually consists of fiber-reinforced thermosetting resin. The intermediate layer contains carbon black particles to permit part absorption and part transmission an incident wave and hence cannot for this reason be the same composition as the outermost layer. Likewise, the innermost layer must be of a different composition having metal or carbon fibers to afford wave reflection. Thus, each of these layers has a different linear expansion coefficient with the result that the finished product upon formation with heat becomes strained, warped or otherwise deformed.